Aircraft windows typically are made from stretched acrylic due to its light weight, flexibility and formability. However, stretched acrylic, like other transparent polymeric substrates, is readily susceptible to wear resulting from contact with various airborne particles (e.g., sand) and liquids (e.g., water), as well as regular contact with atmospheric contaminants and a variety of chemical agents, such as detergents, deicing fluids, and other chemicals used in aircraft maintenance.
Thus, in an effort to increase useful life, aircraft windows typically are treated with abrasion-resistant coatings.
Abrasion-resistant coatings for transparent polymeric substrates have traditionally been polysiloxane-based, polyurethane-based or hybrid sol-gel coatings. These sol-gel coatings are homogeneous mixtures of a solvent, an organosilane, an alkoxide and a catalyst that are processed to form a suitable coating. The sol-gel coatings provide high transmittance and limited durability against wear and ultraviolet induced degradation. The term sol-gel or solution-gelation refers to materials undergoing a series of reactions like hydrolization and condensation. Typically, a metal alkoxide or metal salt hydrolyzes to form a metal hydroxide. The metal hydroxides then condense in solution to form a hybrid organic/inorganic polymer. The ratio of organic to inorganic in the polymer matrix is controlled to maximize the performance for a given application. For example, increasing the organic groups would improve flexibility but may compromise wear durability. The sol-gel coating may include materials such as cerium or titanium to improve abrasion resistance and ultraviolet induced degradation of the coatings.
Such coatings are typically applied using flow-coating techniques, which require significant cure time, limit substrate geometry, and are often susceptible to particulate/dust damage during the curing process. Therefore, flow-coating techniques generally are not amenable to in-service or in-field repair.
Accordingly, those skilled in the art continue to seek new systems and methods for applying abrasion-resistant coatings to transparent polymeric substrates.